In recent years several technical developments have converged to a bigger than ever need for extremely fast data links. High performance computers have become the focus of much attention in the data communications industry. Performance improvements have spawned increasingly data-intensive and high-speed networking applications, such as multimedia and scientific visualization. However, the existing network interconnects between computers and I/O devices are unable to run at the speeds needed.
One approach to provide fast data links, called fibre channel, provides a practical, inexpensive, yet expandable means of quickly transferring data between workstations, mainframes, supercomputers, desktop computers, storage devices, displays and other peripherals. Fibre channel is the general name of an integrated set of standards being developed by the American National Standards Institute (ANSI). Fibre channel attempts to combine the best of channel and network methods of communication into a new I/O interface that meets the needs of both channel users and network users.
Fibre channel allows for an active intelligent interconnection scheme, called a fabric, to connect devices. In a fibre channel system a port simply manages a simple point-to-point connection between itself and the fabric. Since a fibre channel system relies on ports logging in with each other and the fabric, it is irrelevant whether the fabric is a circuit switch, an active hub or a loop. The topology can be selected depending on system performance requirements or packaging options. Possible fibre channel topologies include point-to-point, crosspoint switched or arbitrated loop.
In a fibre channel arbitrated loop, a shared media exists amongst participating end stations and storage devices, for example. In the fibre channel arbitrated loop, a switched media device enables multiple end stations to have simultaneous conversations. To date however, no fibre channel arbitrated loop provides multiple conversations in the arbitrated loops. In these loops with the shared media device, only one pair may have a conversation at a given time.
Existing fibre channel arbitrated loop technologies have limited bandwidth because the bandwidths are shared among all of the participants in the loop. This limits the conversations to only one conversation at a time. As a result, latency occurs within the fibre channel arbitrated loop, which latency can be detrimental to various applications.
A problem associated with latency can be seen in the example of a database application. For example, if a database application is to scale, in the sense that there are more processors in the loop to achieve performance, there is a latency of somewhere in the vicinity of 10-100 microseconds. In a fibre channel arbitrated loop, if one frame time is 20 microseconds, there may be the need to wait for five frames in front of a given frame in order to conduct a conversation. If this occurs, latency extending beyond the 100 microseconds rule occurs, which violates the database application latency limit.
During arbitrated in a fibre channel arbitrated loop, when one individual arbitrates and wins a loop, it sends data to another end station. As long as an end station sends data, it owns the arbitrated loop and no other end station may use the loop. Depending on the amount of data being sent and its data flow rate, the sending station could continuously consume loop resources. This would prevent other end stations from using the loop until the transmission from the end station consuming loop resources was completed. This idea of "fairness" has both to do with how long an end station uses the loop and how frequently the user has access to the loop.
With existing fibre channel arbitrated loop technology, up to 126 devices may be used. This, in most cases, does not occur. Instead most systems use on the order of eight to ten devices. There is no system, however, that practically provides the ability to have multiples of eight to ten devices within an arbitrated loop. Nor is there a device that permits the aggregation of arbitrated loop stations or stations. With the current technology, arbitrated operates only within one loop.